Ionic liquids (ILs) have been the subject of extensive research in recent years and currently represent a new-generation in chemistry. The availability of a variety of cations and anions lend to their unique properties, making them suitable for different applications with potential uses in various market segments, e.g., in the chemical, bio-chemical, pharmaceutical, and technical industries, as solvents, catalysts, electrolytes, or other types of chemicals. ILs also offer considerable efficiency and safety benefits.
ILs have received attention for their abilities to efficiently dissolve and process cellulose, chitin, and other natural biomaterials such as wood, which contains cellulose, hemicelluloses, and lignin (see Swatloski et al., J. Am. Chem. Soc. 2002, 124, 4974-4975; Sun et al., J. Mater. Chem. 2008, 18, 283-290; Qin et al., Green Chem. 2010, 12, 968-971; and Sun et al., Green Chem. 2009, 11, 646-655). The general procedure includes dissolving biomass in IL, then casting the IL-biomass solution on a glass plate and treating it with coagulating solvent (e.g., DI water or ethanol) to form a film; or extruding the IL-biomass solution into a water bath to produce fibers. In either of the procedures, water was mostly used as coagulant (a non solvent for biomass) to help precipitate biomass out of the IL solution and form the desired shape for target end use.
Even though ILs have found a number of industrial applications on biomass processing, several major challenges must be overcome to facilitate the application of the IL technologies into viable commercial process. Among these issues is the current high cost of ILs. Alternative manufacturing and mass-production schemes must be developed to produce an inexpensive IL that meets the desired application performance metrics. Even if the IL price could be sufficiently lowered, the process will need to operate in a closed-loop fashion (i.e., with recycling) to minimize the replenishment of the IL. This necessitates the development and engineering of a process that can efficiently deliver, transfer, and recover the IL.
Recovery of hydrophilic ILs from aqueous solution is highly energy intensive and generally more difficult than that of hydrophobic ILs (see Wu et al., Chem. Eur. J. 2009, 15, 1804-1810). The most common recovery method is to evaporate water out of the system to leave only the IL. However, the direct evaporation process is energy consuming, with much of the energy penalty attributed to boiling water during the IL regeneration, occurring at greater that 100° C. Also, if the IL to be recovered is thermodynamically unstable, such processes should be avoided or minimized.
Liquid-liquid extraction is another commonly used method to recycle catalyst and IL solvent in certain organic reactions. However, recovery has only been demonstrated for some hydrophobic ILs, such as [BF4]− and [PF6]− containing ILs (see Smith et al., Chem. Commun., 2000, 1249-1250; Fukuyama et al., Org. Lett. 2002, 4, 1691-1694). Using organic solvent in liquid-liquid extraction can also diminish the green aspects of ILs. Aqueous biphasic systems (ABS) have been reported to have potential use for recycling hydrophilic ILs from aqueous solution. An ABS forms while adding a water-structuring salt like K3PO4 to an aqueous solution of 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) or 1-allyl-3-methylimidazolium chloride ([Amim]Cl) (see Gutowski et al., J. Am. Chem. Soc. 2003, 125, 6632-6633; Deng et al., J. Chem. Eng. Data, 2009, 54, 2470-2473). The bottom phase is K3PO4-rich while the upper phase is IL-rich, which is mixed with some water and salt. The IL could technically be recovered by drying the upper phase in a vacuum oven and then separating the crystallized K3PO4 by filtration. However, the major obstacle to applying this method into industrial practice is effectively removing the residue salt and water from the IL.
The applicability of an ion exchange mechanism to the enrichment of imidazolium ILs from environmental water samples has been proven using HPLC analysis. The results show it is possible to enrich 1-alkyl- and 1-aryl-3-methylimizadolium ILs on a strong cation exchange resin and furthermore to elute them selectively with developed eluent with yields above 90% (see Stepnowski et al., Anal. Bioanal. Chem. 2005, 381, 189-193). However, research on the sorption behavior of [C4mim]Cl towards a mixed-bed ion exchange resin showed that only 5% of the initially adsorbed [C4mim] cation could be desorbed (see Vijayaraghavan et al., Ind. Eng. Chem. Res. 2009, 48, 7283-7288). Both hydrophilic and hydrophobic ILs are able to be separated from water using CO2 at temperatures between 15-25° C. and pressures below 5.2 MPa. Solutions of water and ILs can be induced to form two liquid phases (IL-rich and water-rich) and one gas phase (mostly CO2 with a small amount of dissolved water) (see Scurto et al., Chem. Commun. 2003, 572-573). However, a complete separation of IL was not achieved using this approach.